The invention pertains generally to leak detection with respect to rotating unions. As used herein, the term “rotating union” refers primarily to a mechanical device used to transfer fluid from a stationary source such as a pipe or hose into a rotating element such as a machine tool spindle or rotating drums found on printing presses and stock calendering machines. A rotating union typically comprises a stationary member, called the housing, that has an inlet port for receiving fluid under pressure, and a rotating member, called a rotor, that has a central passage with an outlet port for delivering fluid into a rotating component. One typical feature of such rotating unions is the ability when working properly to transfer fluid without significant leakage between the stationary and rotating portions.
Rotating unions are used in many industrial settings, including, for example, CNC machining centers, modern printing presses, and other similar industrial environments. The primary usage in such cases is to convey high pressure and/or high volume coolant for use by the process. Coolants used may be water, water-based, or otherwise. The unions and associated equipment taken together can comprise many high precision components such as gears, bearings, couplings, electronic components, etc. that are expensive and/or difficult to replace, and that may be subject to severe corrosion or electric damage if exposed to fluid leaking from the union. In applications wherein the conveyed fluid contains chemical additives, a spill or leakage may present a health risk to operating personnel as well as an environmental hazard.
There are a number of different types of rotating unions on the market. The two general categories are (1) seals that are permanently closed, and (2) so-called “pop-off” seals where the seal may be designed to automatically open the contact between the seal faces when the pressure of the conveyed fluid is absent. Both types are subject to seal wear and eventual failure. The latter type of seal has the advantage of no seal wear in the absence of fluid pressure, but typically exhibits a slight amount of leakage at every shut-down and start-up cycle, such as when automatic tool change occurs in CNC machining systems. For this reason, rotating unions typically incorporate a housing that surrounds the primary seal and one or more drain ports to evacuate the leaked fluid. In addition, rotating unions generally include a back-up seal system between the primary seal (i.e., the seal normally in contact with the conveyed fluid) and any area, such as a bearing chamber, that is to be kept dry. Typical back-up seal systems include one or more labyrinths, air curtains, and lip seals mounted in association with the rotating part of the union. The following U.S. patents describe various details of several types of rotating unions, and are incorporated herein by reference for all that they teach and disclose without exclusion of any portion thereof: U.S. Pat. Nos. 6,164,316; 5,669,636; 5,617,879; 4,976,282; 4,928,997; and 4,817,995.
Once leaked fluid breaches the back-up seal system, the types of damage discussed above often begin to occur. To avoid unnecessary damage, it has long been a goal of manufacturers and users of rotating unions to ensure to the extent possible that rotating unions do not allow excess leakage of fluid. The initial source of such leakage, when it occurs, is the internal seal that provides an interface between rotating (spindle, draw bar, hollow shaft, etc.) and stationary (pipe, tube, hose, etc.) parts while allowing the passage of fluid between the parts. In particular, leakage is typically due to gradual or catastrophic deterioration of this seal. Since, to date, there is no such seal that is not subject to at least eventual wear and replacement, it is important in general to promptly detect leakage within the rotating union when it occurs so that appropriate maintenance may be undertaken before consequential related damage occurs.
At the same time, it is also desirable to minimize the degree to which the leak detecting system gives “false alarms.” That is, if the leak detecting system triggers upon the detection of acceptable levels of leakage, such as may be present during ordinary operation for purposes of lubricating the rotating seal etc., then such system will likely be deactivated or desensitized by operating personnel. This, however, creates a strong risk of eventual undetected harmful leakage.
There have been certain attempts, none completely successful, to solve the aforementioned problems. For example, one type of leak detection system in use as of the date of filing of this application employs a calorimetric sensor situated between the primary seal and the back-up seal system. Other systems appear to employ as of the date of filing of this application a leakage sensor that analyzes the output of the leakage drain port. As will be appreciated from the following description, none of the known existing systems of leak detection provide the necessary level of safety that many embodiments of the present invention are able to provide. In addition, commercially available leakage detection systems are awkward in that their principles of operation and basic configurations force them to rely on extensive external equipment to sense leakage and/or process detection signals.